Topology And Control Strategy Design Of High Voltage Generator For Power Cable Test

With the rapid development of high-voltage and extra high voltage transmission, the power cable industry and research are quickly expanded. Currently, the conventional detection methods of high voltage cable insulation level mainly include two types: high-voltage power cables preventive tests and high voltage power cable fault location. Although related products already exist to meet both routine testing, their size and weight are unable to meet the requirements of portability. In view of this situation and combination of the special electrical characteristics of the high-voltage power cables, this paper introduces two highly portable high-voltage generators which adopt high-frequency inverter technology and digitization method. One is very-low frequency sinusoidal high voltage generator, the other one is high-voltage DC pulse generator. The former is mainly used for the Cross-linked polyethylene (XLPE) cable preventive experimental, the latter not only can be used for high voltage cable fault location but also can be used for routine testing of high pressure oilpaper cable. This paper studies the concepts that related to meet the portability.On the topology of VLF high voltage generator, according to the experimental results and theoretical analysis, a novel topology of very-low frequency sinusoidal high voltage generator is designed in this paper which adopts two high frequency high voltage transformers to reduce the volume of the step-up transformer, generating positive and negative alternating high voltage sine wave. Moreover, multi-drive mode using single transformer is introduced in this paper to reduce the volume of the high voltage bridge obviously, the volume of the100kV/250mA high voltage bridge is only260cm3. On the topology of high-voltage DC pulse generator, by comparing the difference of discharge current loops of the traditional pulsed DC high voltage generator and the high frequency pulse DC high voltage generator, the topology is improved. Double air-gap isolation and cutting off switch tubes strategies which greatly reduce the interference of electromagnetic when sparking, improving the robust of the system work, at the case of the distance between control unit to discharge tube less than10cm, the interference voltage is reduced from1200V to200V. The interference voltage of control unit is reduced from15V to3V.On the Control strategy, this paper adopts series-parallel (LCC) resonant technique to achieve the soft switching of the switch tube, taking full advantage of the high frequency high voltage (HFHV) transformer distributed parameters, reducing the HFHV transformer losses, greatly improving the efficiency of the high-voltage generator. The high voltage generator no-load efficiency is80%, when the load is60%-70%the efficiency reaches94%, the efficiency of the single HFHV transformer is above98%under whole load, greatly reducing the size and weight of the HFHV transformer. In order to cope with the large ranges of output, the operating principle of LCC resonant converter is analyzed in the paper, the static and dynamic analysis is carried on also. Both the converter switching frequency and the duty cycle are utilized as actuating variables to cope with the large ranges of output, improving the response time and robust of the high voltage generator effectively. Step response experiment has been taken on42kV high voltage generator, Step change response time is less than120μs when the output changes from no-load to full-load without any overshoot and ring which verify the accuracy and feasibility of the control theory.On data processing, in order to reduce the calculation complexity significantly, shorting response time, precision control and complying with the wide range of the load, a method named segmental data processing is introduced in the paper for the first time. Each segment contains the control parameters and the error status which can be achieved by looking up the table. By increasing the number of segments, shorter response time and more accurate control can be achieved. The paper describes the principle of segmental data processing method. At the same time, program control flow of the system software and segmental data processing are given.In order to meet the requirements of portability, the volume and the size of the HFHV transformer which is the important part of both of the high voltage generator should been decreased. In this paper, several experiments have been taken on the factors that affect the loss of high frequency high voltage transformer. Through the analysis of the experimental results, this paper points out that the main factors that affect the loss of HFHV transformer is not AC resistance but distributed capacitance. By theoretical analysis of layer-to-layer capacitance of HFHV transformer, a3500W high density HFHV transformer has been successfully designed in laboratory which power density is4.3W/cm3and the weight is only2.5kg, comparing to35kg weight of traditional power frequency transform.Both of high voltage generators are developed for power cable routine test. The test of the35kV/0.1Hz VLF high voltage power supply is taken on a capacitor (1.5μF equal to XLPE cable8.7/10kV/300mm2/5km) which is equivalent to the Cross-linked polyethylene cable in the electrical characteristics. The accuracy of the sinusoidal VLF high voltage is99%and the total harmonic distortion (THD) is less than1%which meets IEEE400.2standard well. Meanwhile,35kV pulse DC high voltage generator is tested by the condition of discharging to ground directly with2μF high voltage capacitor, there is no any abnormal by continue working4hours. The above experimental results verify the feasibility and correctness of the topology and control strategy. The volume of two prototypes is less than0.05m3, and the weight is less than12kg, which can meet the requirement of portability.